Processes for the dimerization of acrylonitrile to essentially straight-chain C.sub.6 dimers are well known in the prior art as illustrated by U.S. Pat. Nos. 4,102,915, 4,126,632 and 4,138,428. Generally speaking, these patents describe processes in which acrylonitrile is contacted with an organic phosphorus (III) compound which has at least one hydrocarbyl and at least one alkoxy or cycloalkoxy group attached to the phosphorus atom or atoms, the acrylonitrile being dissolved in an organic solvent capable of donating protons.
According to most of these teachings, the phosphorus (III) compound has any one of the following formulae: ##STR1## where Ar is an aromatic nucleus, e.g. phenyl or naphthyl, and where groups X, which may be the same or different, are hydrogen or electron donating substituents of the aromatic nucleus which give rise to a negative Hammett .sigma. constant; and R represents an alkyl or cycloalkyl group. Substituents X may be in the para or meta positions; but are preferably para. In the case of compounds of formula (i) above, substituents X may be para in one Ar group but meta in the other.
It will be appreciated that the phosphorus (III) compounds defined above are either phosphinites or phosphonites.
A discussion on Hammett .sigma. constants and a table showing values for most common substituents is to be found in an article by Clark and Perrin in Quarterly Reviews, Vol 18, 1964 pp 295-320.
Examples of suitable substituents X include alkoxy groups, e.g. methoxy, ethoxy, i-propoxy and t-butoxy; alkyl groups, e.g. methyl, ethyl and propyl; and alkyl amino groups, e.g. dimethylamino and diethylamino. The alkoxy, alkyl and alkylamino groups preferably contain from 1 to 8 carbon atoms. It is essential that group X should be one which does not react adversely with the components of the reaction system.
Suitable R groups include alkyl groups such as methyl, ethyl, isopropyl, neopentyl, 2-ethylhexyl; and cycloalkyl groups such as cyclohexyl. It is noted that bulky groups R, for example isopropyl, may give rise to increased catalyst lifetime.
In accordance with these teachings, the presence of an organic solvent is essential to the process because in the absence of such a solvent rapid polymerization occurs. These solvents are said to be "proton-donating solvents" which are substantially unreactive in respect of addition to, or reaction with, the unsaturated linkage of the acrylonitrile or the products of acrylonitrile dimerization. It is taught that the solvent must not react with the phosphorus compounds or catalytic intermediates to form inactive phosphorus species at such a rate as to seriously impair the dimerization reaction.
While the proton donating solvents were not limited to alcohols, alcohols were strongly preferred because they did not react adversely with the phosphorus compound or any intermediates the phosphorus compound may form with the acrylonitrile. Tertiary and secondary alcohols were preferred as for example, t-butylalcohol, 2-butanol and isopropylalcohol.
In order to reduce the amount of hexamer and/or other oligomers and polymers (referred to as polymeric by-products) which were co-produced with the desired dimeric products, it was considered desirable to add a non-hydroxylic co-solvent to the reaction mixture.
These non-hydroxylic organic solvents included hydrocarbons such as hexane, cyclohexane, benzene, toluene, and petroleum ethers; ethers, such as tetrahydrofuran, diethyl ether and diisopropyl ether; nitriles, such as acetonitrile, propionitrile and fluorobenzenes. The hydrocarbon co-solvents were generally preferred.
In accordance with prior art processes, the desired products were readily separated from the reaction mixtures, for example, by fractional distillation or solvent extraction.